1. Field of the Invention
The present invention relates generally to gas turbine engine combustor inlet diffusers and, more specifically, to blowing air into boundary layers of annular walls surrounding diffuser flowpath.
2. Background Art
A conventional gas turbine engine includes in serial flow communication, a compressor, a discharge flowpath having a stage of inlet compressor outlet guide vanes (OGVs), disposed between annular inner and outer walls, which in turn are mounted in an OGV support structure mechanically tied into an engine casing. Outlet guide vanes typically have airfoil like cross-sections that include a leading edge, a relatively thick middle section, and a thin trailing edge. Downstream of the OGVs is a combustor inlet diffuser, a combustor, a turbine nozzle, and a high pressure turbine. Typically, OGV inner and outer walls are supported by corresponding inner and outer annular diffuser inlet walls to form a relatively leak-free flowpath therebetween and support the OGVs and diffuser. The OGVs, inner and outer walls, and diffuser may be a single piece, integrally cast assembly or in some other constructions corresponding inner and outer OGV walls with the OGVs therebetween are welded to a diffuser casing.
During engine operation, the compressor compresses inlet airflow, which is therefore heated thereby. The discharged compressed and heated airflow is then channeled through the OGVs and the diffuser to the combustor wherein it is conventionally mixed with fuel and ignited to form combustion gases. The combustion gases are channeled through the turbine nozzle to the high pressure turbine which extracts energy therefrom for rotating and powering the compressor.
Typically, the high pressure air at the compressor exit is conditioned to have low swirl and low Mach number for use in the combustor and the outlet guide vanes and diffuser are employed to condition the compressor discharge air to be suitable for the combustor. Some engine configurations also require the OGVs to serve as a structural member which places additional constraints on the design. Conventionally, outlet guide vanes reside in a constant annulus height flowpath. The flowpath may help turn the flow radially outwardly to help align it with the downstream combustor. The OGVs are designed to remove tangential swirl from the compressor discharge air so that upon leaving the OGVs air flows nominally in the axial direction. In the process of deswirling, the flow's tangential momentum is converted to static pressure, reducing the flow's absolute Mach number. The diffuser defines a diffuser flowpath downstream of the OGV trailing edge, which further decreases the flow Mach number by one or by a plurality of diverging annular passages. These passages may also guide the flow radially outwardly, providing yet, more diffusion for a given annulus height. Adequate efficiency and stall margin are obtained by employing sufficient airfoil solidity, selecting proper airfoil incidence, optimizing the surface velocity distributions, and providing enough diffuser length/area ratio to avoid flow separation.
It is desirable to supply high pressure compressor exit air to the combustor as efficiently as possible with sufficient stall margin while minimizing engine length and hence weight and cost. Reduced length typically results in higher diffusion rates which makes the boundary layers more susceptible to separation which negatively impact performance and stall margin. Thus, reduced length and high diffusion rates tend to be conflicting requirements. It is desirable to reduce the axial length required to deliver this air and hence to reduce engine length, weight, and cost while maintaining performance and stall margin.
New gas turbine engine designs have been proposed employing advanced compressors that operate with very high compressor exit Mach numbers. At sea level take off conditions, the compressor exit Mach number may be as high as 0.45, with a dynamic velocity head of about 12.5 percent of the total pressure. Conventional combustor inlet diffusers, designed for these conditions, have high pressure losses, which would cause a considerable increase in engine specific fuel consumption. To minimize these losses, the diffuser must recover as much of this velocity head as possible. A very long conventional diffuser may recover as much as one-half of this velocity head, but the pressure losses would still be high and the engine would be considerably longer and heavier. Short length, low pressure loss diffuser designs are needed for these advanced engine applications.
One proposed approach to solve this problem is to use boundary layer bleed on the outer and inner walls of the diffuser to prevent flow separation in short length, high area ratio diffusers. However, bleed diffusers require the removal of 8 to 12 percent of the compressor exit flow for good diffuser performance. For good engine performance, this flow must be reintroduced into the engine with minimum pressure losses. Some of this flow could be used for turbine cooling, but at this point in the engine cycle, the pressure is considerably lower than the compressor exit pressure, which would result in sizable pressure losses for the bleed flow.
It is highly desirable in the gas turbine engine industry and, particularly, in the aircraft gas turbine engine industry to design and build short combustor inlet diffusers. In order to do this, it is desirable to build such diffusers with apparatus that prevents or delays separation of the boundary layer in an efficient manner.